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In this world:

Technology: medieval, it should fit with real world history, for example:

  • Low life life expectancy due to poor medical technology (vulnerable to disease), warfare,
  • High infant mortality, especially for the poor.
  • ...

In this world, there are nobles and non-nobles (peasants).

Condition to be noble:

  • Biological parents (both mother and father) are noble
  • If not sure, assume to be peasant.
  • The first nobles were promoted from the peasantry by the king and divine entities are suggested in folk tales. However, it has been a long time since that happened. In all official history records, there are no named peasants who were promoted to be noble.

Duty:

  • Nobles must protect their subjects (including peasants, and other nobles). It is stated to be more important for a duke to protect his nobles vassals than his peasants.
  • Peasants are protected by their lord. However, this is the matter of the lord's honor (he who fails to protect his subjects is disgraced) and fortune (peasants work for their lord on his land). He should able to protect most of his peasants in time of crisis, but never risk his life for a single peasant life. However, the life of an single peasant is not important as they are expendable (peasants are likely to be punished by death, peasants in war as cannon fodder, etc.).

Marriages:

  • Marriages requires a man and a woman (same sex marriage is forbidden by divine command).
  • Wife and husband can be both nobles, both peasants, or a peasant and a noble.

As you see, the society is structured so that the nobles live a luxury life provided by the peasantry. It is better to be a noble.

However, due to the condition of being a noble, if a noble person marries a peasant, their offspring are always peasants. Thus, in peasant bloodlines, there are noble bloodlines, but only rarely. Meanwhile, by definition, all nobles have purely noble bloodlines. This discourages nobles from marrying peasants because their offspring would no longer be noble.

  • I would like to ask whether it's possible for nobles and peasants to become 2 sub-species of human that cannot mate together to make offspring ?

  • If so, how many generations would it take?

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    $\begingroup$ If the relatively small population of nobles want to "maintain pure bloodlines" long enough to make a new species, some really nasty genetic consequences will ensue. On one hand, those recessive genes being expressed frequently may create a new species sooner. On the other, nobles will be plagued by health problems. Check out the Habsburgs of Spain. en.wikipedia.org/wiki/Charles_II_of_Spain $\endgroup$ – Zxyrra Jan 20 at 19:35
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    $\begingroup$ @Zxyrra excellent point. I read somewhere you need at least a pool of 40k plus individuals in order to have a viable gene pool without nasty inbreeding side effects. $\endgroup$ – Paul TIKI Jan 20 at 21:10
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    $\begingroup$ The correlation between marriage and biological descent is tenuous at best. Moreover, in the real Middle Ages, illegitimate sons quite often inherited the titles and positions of their noble fathers, whathever the Church and the law might have had to say. (The Middle Ages were not a good time for the rule of law.) For example, you may have heard of the rather famous William the Conqueror, who was known as William the Bastard in his time (and, yes, even to his face). An illegitimate son is infinitely better than no so at all... $\endgroup$ – AlexP Jan 20 at 22:12
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    $\begingroup$ Without DNA tests there might have been many "illegitimate" children nobody (or nobody except the mother) knew were really illegitimate, especially if it was the mother who cheated. Cue to the old joke of the stablehand cursing under his breath "my father is a baron, my son is a baron, and I still have to work in this ugly stable." $\endgroup$ – vsz Jan 21 at 7:35
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    $\begingroup$ Just a short comment that may be sligntly off-topic: "Low life life expectancy due to poor medical technology (vulnerable to disease), warfare" This is not necessarily realistic as the "low live expectancy" of historical periods refers to an average and applies to high child_mortality. Adults would live to the same age. Same for "Warfare": War was not really common, it's just that we are talking about a very long period and history only lists the "interesting" parts, thus wars and similar. War were actually much more localized and restricted than nowadays. $\endgroup$ – runlevel0 Jan 21 at 12:17

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This speciation will never occur.

Why? A condition Robert A. Heinlein once described as "common bastardy."

People don't always keep their genes to themselves. Humans are well known as what I think of as "false monogamists" even in what are considered monogamous societies. There has never been a noble class in human history that didn't "indulge itself" with the lower classes -- and indulgence of the nature I'm talking about leads to the occasional accidental pregnancy.

This isn't always the duke or king producing offspring on the wrong side of the blanket, as it were, either. There are a number of well documented historical cases of noblewomen producing children that, for one reason or another, could not have been "legitimate."

It only takes a tiny number in each generation resulting from such interbreeding to keep two otherwise separate populations within the same species. This is why humans aren't multiple species today: even those who (like Australian natives) were isolated from other humans for thousands of years (because they weren't; there was surely some interbreeding with people from what became Malaysia and Melanesia).

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    $\begingroup$ Yeah, if your plans rely on some or more humans keeping their pants on, then your plans are doomed to failure, and this has been the case for millions of years. Ever wonder why the closest relative of the pubic louse is a gorilla parasite? Wikipedia suggests early humans may have "slept in gorrila nests". I suspect there's a "nudge nudge, wink wink" editted out of there. $\endgroup$ – Starfish Prime Jan 20 at 19:58
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    $\begingroup$ Apropos of which, I find it rather amusing to compare the appearance of (former?) Prince Harry and the rest of the males in the British royal family. $\endgroup$ – jamesqf Jan 21 at 5:36
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    $\begingroup$ There is a joke about a jew coming to his Rabbi and complaining that his wife is having sex with the count. The rabbi answers: "But Moshe? Why do you complain? Everyone knows you have sex with countess!". - Yes, but I make him little dukes and he's making me little Moshes". $\endgroup$ – SZCZERZO KŁY Jan 21 at 10:28
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    $\begingroup$ @ZeissIkon : let's not forget one of the most common causes of massive gene spreading in historical periods and directly related to war: Rape. This was a pretty common practice and to some extent it still is (unfortunately). $\endgroup$ – runlevel0 Jan 21 at 12:24
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    $\begingroup$ By the time of the novel, the "breeding program" (nobles marrying other nobles, and the most important families having the greatest psychic strength) had been going on for many centuries. But there was no suggestion that "nobles" and "commoners" would someday be separate species. On the contrary, with lots of commoner girls wanting to get pregnant with psychically gifted kids, the implication was that someday every human in that world would have psychic powers to some degree, as the powers kept spreading throughout the gene pool. $\endgroup$ – Lorendiac Jan 21 at 14:53
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Longer Than the Medieval Period Would Last

Speciation takes a long time, save in extraordinary genetic situations (polyploidy, for instance, can create non-interbreeding hybrids in a single generation, but only in plants). Even as breeds start to diverge, interbreeding is still often possible, making definitive speciation (and definitely the kind you're talking about) difficult to call.

The only human example we have to draw upon is the association between Neanderthals and modern humans. In that case, you had two distinct species from the same common ancestor who gradually lost the ability to interbreed. That took, at minimum, a hundred thousand years.

This is a longer interval than human recorded history, and certainly longer than any noble/non-noble marriage rules would last.

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    $\begingroup$ There's now evidence that Neanderthals, Denisovans, Cro-Magnon (us!) and a fourth, yet-unnamed species of hominin never lost the ability to interbreed before the other species became extinct. $\endgroup$ – Zeiss Ikon Jan 20 at 18:51
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    $\begingroup$ Yes the homo spaiens genome contains Neanderthal DNA so there was inter breeding. Whether the Homo Sapiens and Neanderthals count as separate species depends on which definition of species you use. $\endgroup$ – Slarty Jan 20 at 18:53
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    $\begingroup$ @Slarty - we also have chimpanzee DNA, so I wouldn't argue that common DNA means interbreeding continued. That said, all that needs to be true for the answer is that a hundred thousand years wasn't enough for interbreeding to become impossible. $\endgroup$ – jdunlop Jan 20 at 19:00
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    $\begingroup$ @jdunlop Seems to me (based on genetic dating) there is evidence that the most recent interbreeding with Denisovan and Neanderthal is much more recent than that, as little as 30-40k years. Still far longer than any reasonable (i.e. not artificially maintained from outside) medieval period could last. $\endgroup$ – Zeiss Ikon Jan 20 at 20:22
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    $\begingroup$ Speciation usually takes a long time, but it doesn't have to. In fact, speciation has been observed within a few dozen generations in experimental evolution: pnas.org/content/116/27/13440.short $\endgroup$ – Jack Aidley Jan 21 at 9:54
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Horses and Donkeys can mate to produce viable (if infertile in almost every case) offspring. The last common ancestor living approximately 4 million years ago. Taking a generation as 2 years that gives 2 million generations as a starting estimate.

The generation time might be a little longer depending on circumstances and the time period should be a little shorter if fertile off spring are needed so say 100 thousand - 1 million generations to be on the safe side. And yes I know humans are not horses or donkeys but the same sort of genetic drift should apply to isolated groups.

That said it would seem from the situation that you describe, that the noble stock might dwindle to nothing as the population is drawn off to the peasant side.

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    $\begingroup$ Camels and llamas produce fertile offspring together despite last having a common ancestor 11 million years ago link, and bottlenose dolphin and false killer whales can produce fertile offspring ("wholfins") despite having last shared a common ancestor about 8 million years ago link. Both of these groups can interbreed despite really large morphological differences. $\endgroup$ – user2352714 Jan 20 at 20:59
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    $\begingroup$ Interesting. So it might take even longer. That said there must be a lott of other factors involved so years since last common ancestor is only a very crude estimator of the likelyhood of breeding being possible $\endgroup$ – Slarty Jan 21 at 0:22
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    $\begingroup$ @Aganju 2 years is a fine approximation for horse generations $\endgroup$ – Nolimon Jan 21 at 14:24
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    $\begingroup$ Not to take the discussion too far afield, but it should be noted that these hybrids are only possible with specific combinations of maternal and paternal species, and not all are viable, so the two animals cannot be considered the same species. More interesting results are seen elsewhere in the taxa, for instance in the "caniforms" like bears and dogs, the extants of which have more recent genetic splits from sister species (115-250kya). This jives with the consensus that about 100,000 generations, which for humans would be about 20 million years, is required for a species split. $\endgroup$ – KeithS Jan 21 at 20:09
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    $\begingroup$ Found another article that uses "reproductively viable", which comforts me slightly ... $\endgroup$ – Ross Presser Feb 27 at 15:55
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Consanguity can rescue infertility due to a balanced translocation. It could be a speciation event.

Here is a balanced translocation in a healthy person, and an unbalanced translocation in his unfortunate progeny.

chromosomes and unbalanced translocation

http://infertilefarmer.blogspot.com/p/what-is-balanced-translocation.html

Balanced translocation is a common reason for infertility. The parent (depicted on top) has had a piece of one chromosome swap places with a piece of another. You need the entirety of the material from both chromosomes and he has it, just not in the regular places.

But when a sperm carries half of his chromosomes off to meet and egg and make a baby, it is likely you wind up with the unbalanced scenario. 3/4 of the time you are missing a piece and the fetus has an unbalanced translocation. The mother has only unicolor chromosomes.

If the mother has the same unbalanced translocation (because they are cousins) then the chance of success goes up. If dad contributes yellow with purple tip and mom contributes purple with yellow tip, all is well. The chance of conceiving is actually theoretically higher with the consanguineous union!

With more than one event of this type, the chance of a person with multiple balanced translocations conceiving with anyone other than close family drops considerably.

Now in the real world there are other problems with consanguineous unions / inbreeding and all of those would hold true. Persons interested in real science: do not interpret this to mean that you should produce children with close family members. But for a fiction and a method to cause consanguineous nobles to speciate out, this could happen in just a few generations.

For those tempted to call B.S. - read this.
Preimplantation genetic diagnosis (PGD) for extremes--successful birth after PGD for a consanguineous couple carrying an identical balanced reciprocal translocation. Technically skilled fertility doctors pulled off the above described feat and allowed successful birth of phenotypically normal babies from consanguineous parents carrying the same balanced translocation. The babies of course carry the same balanced translocations (they have to!) and so would themselves have the same problem as their parents.

Chromosomal rearrangements (CRs) can definitely lead to speciation events!

Chromosomal Speciation in the Genomics Era: Disentangling Phylogenetic Evolution of Rock-wallabies

If, however, CRs generate beneficial fitness effects (spread by positive selection), we do not expect fixation to occur at similar times. With combined cytogenetic understanding, this allows us to fit models to different regions along each chromosome to capture their unique evolutionary histories. If rearrangements are important to divergence, we expect the times at which they are established to coincide with speciation events.

For your fiction, have the rearranged chromosome confer something adaptive for the nobility. They are better than the peasantry for some genetic reason - maybe immunologic, or neurologic, or magical.

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    $\begingroup$ Very nice answer, but it seems like the adaptively rearranged chromosome would speiciate better if it happens in peasants since the odds of breedable pairs splitting off from nobility into the commoners is much higher than vise versa in this scenario. Otherwise, it would probably have to have an effect that is only selectively fit in one group. Such as a massive boost in metabolism that starves out poor peasants, but turns well fed nobles into stronger warriors. $\endgroup$ – Nosajimiki Jan 20 at 21:27
  • $\begingroup$ Basically your answer boils down to given the right circumstances it might happen. But doesn't mean that class segregation, in & of itself, will lead to speciation. Only if you lucky. $\endgroup$ – a4android Jan 21 at 0:53
  • $\begingroup$ @Nosajimiki-ReinstateMonica Sure, but simple chance could mean it happened to the nobles. And you need two such events--one that sets up the situation and spreads through enough of the noble population and then one that can only breed with someone who has the first stage translocation. $\endgroup$ – Loren Pechtel Jan 21 at 17:25
  • $\begingroup$ It seems like a particular mindset in the nobles might lead them to seek out such a solution and implement it artificially. "We want our kids to become infertile with the peasants. Genetic engineering during IVF for everyone!" $\endgroup$ – Ross Presser Jan 21 at 21:59
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    $\begingroup$ One minor addition, is that the adaptation is for some social ritual that only the nobility partake in. Any bastard children are spotted in this public ritual and outed and sent back into the peasants. This means the adaptation only needs to be better for the ritual and doesn't need to be generally adaptive in a way that would benefit non-nobles. $\endgroup$ – Pureferret Jan 23 at 11:10
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TL;DR: Based on the genetic progression we have observed in ourselves and other species thus far, at least 300,000 years and possibly more than 700,000 years of strict genetic segregation would be required for humans to see an irreconcilable genetic difference between genetically-segregated humans, heralding the creation of a truly distinct species of humans. The only way such a genetic divide could have developed during a period of maybe a millenia or two of feudal rule is for some more acute reproductive change to have occurred within one population or the other.


Speciation, the process of one species becoming genetically distinct from its parent and from any "sister species", thus unable to freely interbreed, takes quite a while. The most recently-diverged genii we are aware of are Ursus ("true" bears including black, brown and polar), Canus (dogs, wolves and coyotes), Vulpes (foxes), Panthera (large cats) and Felis (small cats) genii. All of these show evidence of easy hybridization among species within the genus, but they cannot freely interbreed to produce viable offspring in all combinations of mother and father and so they are indeed separate species.

Which is the first problem with your posit; even if humans do diverge genetically across social strata, it's very likely that viable offspring will still be possible in at least one combination of species and sex for many millenia after that, much as we see among relatively closely-related genii. Those hybrids, whichever society they mingle with, will infuse their genetic material gained from the other group through several generations of their descendants, which will help perpetuate the close genetic relationship between species and, if these trysts are common enough, even prompt a re-merging of the species by introducing enough of what makes one population genetically incompatible into the DNA of the other population, until some critical mass is reached that allows free interbreeding again.

The second problem is that even the most recently diverged genus, Canus, split into its extant sister species between 50,000 and 115,000 years ago. That represents about half that many generations depending on specific species and behavior, with female wolves sexually mature in a year, but not commonly leaving their birth pack until about 2-3 years of age as they're courted by unrelated males to become alphas. Extrapolating that 25,000-generation minimum to humans, with a roughly 20-year maturation period to a more socially-defined marriageable age, we'd expect even the weakest degree of speciation, losing at least one combination of parentage to nonviability, to occur over a span no shorter than 500,000 years of strict separation of the genetic stock, giving the genetic webs 25,000 generations of separation from any common ancestor. Even using the onset of female puberty (about 12 years of age) as the age of maturity and thus the minimum span between genetic generations, we're still talking about 300,000 years of genetic separation.

Now, that's a minimum timeframe. It also represents about the sum total of Homo sapiens' existence on this planet. We know that before Neanderthal man was out-competed by anatomically modern humans spreading from the Mesopotamian region about 50,000 years ago, Neanderthals and modern humans intermingled, with every non-African population of humans retaining about 2.8% of the Neanderthal genome. That genetic mix represents a confluence of DNA previously separated by as much as 700,000 years, when the genetic branch that resulted in Neanderthals first split from our own about 860kya. Whether all combinations of Neanderthal and modern human, male and female, produced viable offspring (and therefore H. neanderthalensis and H. sapiens were still genetically the same species) is unknown, but it's unlikely, given the proliferation of Neanderthal DNA in the human genome, that this was a rare occurrence.

So, while 25,000 generations is a minimum timeframe, giving us a neighborhood of between 300k and 500k years to start seeing genetic speciation happen in genetically-segregated humans, we also have archaeological evidence that anatomically modern humans didn't truly segregate from Neanderthal cousins over a timeframe up to twice that long. At the upper end of canine speciation estimates and human maturities, we might expect genetically-segregated humans to truly speciate on a timescale of a million years (but still produce viable hybrids from the male of one species and the female of the other). In any case, expecting it to happen naturally as the result of even a couple millenia of social segregation between cohabiting human populations just doesn't pass the smell test.


So, the only feasible scenario for your worldbuilding is that some more acute genetic mutation has rendered the nobility incapable of producing viable offspring with the rest of humanity. This mutation could be environmental in nature, or could arise quite simply by inbreeding among a small genetic population.

The first device is a fairly easy handwave and it's been done before. In the video game Freelancer, the Outcasts are the descendants of one tribe of the doomed sleeper ship Hispania, and landed on a planet with a predominant form of plantlife that ended up altering the Outcasts at a genetic level, making them unable to tolerate being away from the Outcast homeworld Malta for any extended period without a supply of an extract of the plant, called cardamine.

In another example, the Divergent Series novels and movies ultimately reveal Triss's home city of Chicago to be an experiment by a far more advanced branch of humanity that survived a global war. Chicago's inhabitants were genetically damaged by weapons used in that war, limiting their inherent human traits, and the experiment, run by the descendants of humanity who escaped such weapons, sought to find out whether the genetic damage would naturally heal over time. Divergents, ostracised and hunted down as undesirable by Chicago's leadership (totally unaware of the experiment), turn out to be exactly what the experiment was intended to produce, as individuals' genomes repaired themselves over generations. Reproductive difficulties weren't covered specifically, but this is an obvious direction to take an underlying story about a society of genetically-damaged individuals.

The second device, simple inbreeding, has pretty sound basis in our own reality. Estimates of the "minimum viable population" of a genetically random or localized population hover around 4,000 individuals; any fewer than that and you will, given sufficient time, see symptoms of inbreeding caused by insufficient genetic diversity. You can make do with a smaller population if that population is more genetically diverse, down to a minimum hypothetical "ark population" of approximately 500 individuals, specifically selected from across the entire human species to maximize diversity across the genome, and then the proper sequence of reproductive pairings "arranged" through each generation, stud-book style, to disseminate that genetic diversity as efficiently as possible.

As of the creation of Great Britain in 1707 by the formal merging of the crowns of England and Scotland (which had rested on the same head since James VI of Scotland inherited the English crown from his cousin Elizabeth I in 1603 to become "James VI and I"), the "peerage" or nobility of both predecessor nations numbered only 322 individuals and their extended families. Let's say the average family size, including only children surviving to have their own children, was 5; that's only 1500 individuals in a roughly steady-state population, many of which could likely already trace their familial relationship to most of the rest of the peerage within four degrees of blood or marriage on both sides of their family tree, well below the 4,000 minimum viability of a random localized genetic sampling.

It's a long-running joke that the English nobility have kept it in the family just a few generations too many, and while both the sons of Charles and Diana have married and had children outside the nobility, Prince George (William and Kate's eldest son) is the first person in line to the British throne that I can find since the Tudor era (Edward VI, son of Jane Seymour) to not have two parents of noble birth.

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It’s “just” a matter of random mutations. It’s highly unlikely but if it’s important for your story you could have it happen within a few generations. The nobles could even point out how special and unlikely it is and that it’s certainly a sign of divine benediction.

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  • $\begingroup$ For it to happen within a few generations, you do need all the nobles to be descendent from one mutated individual within a few generations. Might be possible, but a challenge that one has to keep in mind. $\endgroup$ – Mark Jan 21 at 11:40
  • $\begingroup$ Actually come to think of it, if the mutation prevents creating offspring with non-mutated individuals, then how would that mutation spread at all? $\endgroup$ – Mark Jan 21 at 11:43
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    $\begingroup$ @Mark: I don't know but it obviously works, somehow there is more than species on this earth. $\endgroup$ – Christian Jan 21 at 12:03
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    $\begingroup$ @Christian I assume those are created by groups slowly drifting apart while still reproducing within the group. I assume that if one individual mutates into a new species, he'll have no offspring. $\endgroup$ – Mark Jan 21 at 12:05
  • $\begingroup$ @Mark: Good point(s). Maybe strong selection for the “new” species would help to accelerate its propagation? For example some kind of plague. There are theories that the Black Death and other diseases actually affected the genetics of humans in certain geographic areas. $\endgroup$ – Michael Jan 21 at 12:22
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In medieval terms.. the children of peasants and nobles always fail to thrive; you don't need any more detail than that.

It could be caused by the noble females, being closely related, all carrying an incredibly rare mitochondrial mutation x, being a mitochondrial mutation it is passed unchanged to all of their offspring.

Some noble males carry an equally rare mutation y.

Inheriting x without y will lead to the offspring being infertile, or not surviving to adulthood. A noblewoman can then never, successfully, have a peasant's child.

Some noble-noble offspring would fail, because they may not inherit y from their fathers (~50%) all noble-peasant offspring would die.

You would also have to make mutation y fatal on it's own, to close the loop and prevent peasant-noble children from inheriting it and passing it on.

Depending on how the original nobles were selected, this could have happened from day 1. If the original noblewomen were selected form a specific group or tribe (or maybe they were from a foreign land, due to the otherworldliness) they may have all carried this mutation which was not present in the native population

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Without evolutionary pressure, the answer is never. Evolutionary change comes from random mutations plus an environmental factor that selects for that change. If random mutations happen within individual nobles and peasants, but nature doesn't effectively favor these individuals, then these mutations won't become "norm" in the population. If no large changes happen in the populations, then there's no reason to think their reproductive systems will become cross-incompatible.

Nature is rife with evolutionary pressure, as species fight for resources, migration brings new competitors in, weather patters change over thousands of years. Human society, even a medieval one, doesn't have nearly as much pressure. Societal factors (who you were born to, what you receive for education, etc) determine your breeding patterns, not being on average bigger or better.

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In reality: not in a time scale that is compatible with any sort of society we're familiar with. Others have covered this well, I have nothing to add on that.

There are workarounds for this, for example you could invoke divine favor to keep this society going for a Really Long Time. As long as you're not trying to maintain real-world astrophysics and follow the recipes in How To Build a Habitable Planet and generally maintain nerd-compliant reality, there's plenty of precedent for an "it's always been there" society, and this can be a reasonable occasion for suspension of disbelief. You've already established that there's a deity on the scene, so we're already outside of nerd-compliant reality anyway. This has the added advantage of offering infinite scope for Other Tales.

However, there is one basic problem that I think torpedos the idea for me, and that is that it's plot-wise inconsistent with your setup, under "nobles and peasants marry and produce offspring" is part of the background.

I don't see how you can preserve this in your setup and still get the speciation, without invoking a lot more magic than I'd be willing to tolerate under "suspension of disbelief".

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Ruddy ducks!

For one animal species, the answer is well in excess of ten million years, and that's a minimum. It might be as much as 50 million years during which it didn't happen.

When the Atlantic Ocean opened, it separated two populations of ducks. One evolved into the North American Ruddy Duck, the other into the European White-headed duck. They have completely different plumage, and were classified as different species.

But when humans unwisely introduced Ruddy ducks into Europe, they immediately started breeding! I have read that female white-headed ducks actually preferred ruddy ducks as mates. Humans have decided that such hybridisation is not a good idea and have shot the ruddy duck and obvious hybrid ducks in Europe to extinction, although it's virtually certain that some ruddy duck genes have entered the gene pool of the white-headed ducks.

The strongest definition of species is that the organisms cannot interbreed and produce healthy fertile offspring are separate species. A lesser definition is that two species choose not to, or simply can't for geographical reasons. This was a case of can't, because of the width of the ocean. As soon as humans bridged it for them, they did, with enthusiasm!

https://www.rspb.org.uk/our-work/our-positions-and-casework/our-positions/species/invasive-non-native-species/ruddy-ducks-and-white-headed-ducks/

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Speciation requires two things to happen: firstly a new gene in one individual of Population A that prevents successful fertilisation with Population B, and perhaps other genes conferring survival success. Then a period while the new gene becomes established in population A.

The first takes no time at all, so the second is the limiting factor time-wise. If we assume ten surviving children per couple, every generation, 8 generations would get us to 100 million, which is enough to be called "established" in most populations, though there might still be a few individuals that could interbreed.

So assuming 20 years per generation, we are only talking about 160 years. The probability of this happening is extremely remote, but definitely possible. If you want something that is more likely, it would require more time, but is always going to require that first mutation.

Absolutely nothing to do with species (e.g dogs and wolves) where this has NOT happened.

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The speed of the "speciacion" depends on both mutation pressure, selection pressure and the quality of the separation between populations.

As per your scheme, peasant population is not insulated by definition. Medieval science and technology (and human nature in general) imply that no actual control of gene transfer can be implemented. In the wild, it takes a tall mountain or clima belt to separate populations of plants or animals. So long for separation.

Selection pressure difference: we can assume there is little to none in the nobble and at best medicore in the peasants. Common factors (e.g. diseases) will affect equally both. No luck there either.

Mutation pressure: who knows. No ionizing radiation, probably some poisons used by the nobble and that's all for medieval setup.

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Pockets of humanity have been cut off from contact from others for thousands of years at a time and this physical separation between populations did not result in speciation.

The administrative separation within your populations would not have a more marked effect than an actual physical separation.

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The normal process of speciation tends to require:

  • A relatively small population.
  • ..that is isolated from interaction with surrounding populations
  • ..that has significant divergent selection mechanisms acting on it.

Google "punctuated equilibrium" evolution including the quotes.

Look at dogs. We've had dogs with us for tens of thousands of years, and have been imposing our selections on them for a few thousand years.

But all dogs are still considered the same species, and as far as I know they can all interbreed.

Dogs, wolves, and coyotes are considered separate species, although they can successfully interbreed.

Anyway, with domestic dogs, you would typically have a selected litter (after proving your worth to your owner) probably an average of 3 yr per generation. If we suppose serious selection only from the time we were able to pen the females separately when in heat, call it, what, 4000 years. So with 1000 to 2000 generations (lots of handwavium) we don't have separate species, although the mechanics of breeding chihuahuas and great danes would be interesting.

I think you could make a case for wheat being a different species from the grain it started from, ditto corn. On the other hand, cabbage, kale, broccoli, cauliflower, brussel sprouts, and kohlrabi are all the same species.

Humans tend to see well formed strangers as attractive. The exotic look. There is a strong drive to hybridize.

The longest case I can think of right now of population isolation is that of Australia. People were separated for at least 40K years, from the rest of the world. But perhaps the population was too large for genetic drift to occur.

If you want to make this plausible, you need some agency putting strong selection pressure, and isolation pressure between populations, but this requires a degree of institutional permanence that humanity hasn't exhibited. From the examples above, I'd estimate 10,000 generations at a minimum to get to the non-viability of hybrids.

  • Your culture is the property of a very long lived alien. It acts like a dog breeder when his best bitch has been bred to a mongrel: Drown the pups.

  • There are gods. (Aliens with a different label)

  • There is magic. A compulsion that forbids certain unions.

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We can try and do this in reverse.

First stop them from interbreeding, then have them evolve differently (sympatric speciation).

We cheat, introducing devilgrass. This is a very nourishing, sturdy perennial plant that is almost ubiquitous (an infestant, actually); it was rarely used as food since it's also poisonous, and requires special cooking to be digestible. Even so, it can trigger a case of the runs, and worst case, impairs platelet activity in the body, resulting in haemorraging.

But during harsh winters there is often no other easy resource. So, in the dim past, people started eating devilgrass (that's how the special cooking was discovered).

Then a random, recessive mutation happened that changed the cyclooxigenase mechanism in some peasant. As a result, they became partially immune to the devilgrass poison. The mutation evolved, but it never "took" among the nobles, since no noble can have a peasant ancestor. Villages where the mutation ruled routinely had a population excess, so naturally their Devilgrass-Resistant (D/R) population tended to spread to nearby villages, even colonizing wholesale whenever famine and sickness had led to some of those being abandoned.

Within thirty reproductive cycles, say one thousand years, a large area has D/R peasants only: as soon as the D/R ratio rises too much, devilgrass cooking stops, which forces D/S peasants to flee, die, spend for healthier fare, or devote much more time than the new normal to cooking.

Now the mutation has evolved to the point that the "old" and "new" style ciclooxygenase mechanisms begin interfering. There are no less than five different COX alleles around, Noble0 and Peasant1 through Peasant4. The larger the index, the more marked the devilgrass immunity (the Peasant1 allele is almost extinct even though it spread the farthest, replaced almost everywhere by at least the P2); and the larger the difference between the indexes, the higher the chances of a spontaneous abortion. P44 children are healthy and always come to term. Most P41 die stillborn. The children of a noble and a P1 almost always survive, but won't live long because they need specially cooked food. The children of a noble and a P4 never come to term.

It will take anywhere between twenty and one hundred centuries even so (you could greatly accelerate times with some unwitting eugenics - say some religion preaching that devilgrass sensitivity is a sign of impurity and evil unless one is a noble, in which case the reverse applies. So you should never, ever marry a D/S individual, or fight to allow a D/S child survive. God wishes it so).

But in the end, a noble might take a peasant lover and neither should worry about a pregnancy (again, God wishes it so).

The two "species" were always different however: the different diet ensured nobles were routinely taller and straighter than peasants (the same difference exists between moderns and ancient Romans). Now they can start diverging in earnest.

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H.G. Wells' novel The Time Machine. Made into a movie by George Pal. In the movie, some humans went down into bomb shelters and learned to survive there, while others stayed up top, and they evolved to be pretty different

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  • $\begingroup$ Welcome to Worldbuilding, Wood. In Wells' novel the Eloi and the Morlocks had 800,00 years to differentiate. They may have still been the same species. Wells didn't discuss their mating or breeding habits, so we really don't know for sure. My inner biologist believes more time would be necessary for speciation to occur. A lot more time. Plus a few other evolutionary factors. $\endgroup$ – a4android Jan 23 at 8:31
  • $\begingroup$ 800,000 years? Humans split from Neanderthals in less than that time. $\endgroup$ – Oddthinking Jan 24 at 1:33
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The Japanese novel Shin Sekai Yori (eng: From the New World) (2008) covers this rather graphically and specifically. Starting from (apparently) random mutations in the late 20st century, a psychic overclass develops after non-psychic (or degenerate-psychic) humans are systematically murdered and/or altered physically to new, animalistic forms. The "present-day" of the story is set about 3000 years in the future, but even then, even with active and purposeful social, genetic, & physiological manipulation, the underclass and overclass are still genetically a single species.

Octavia Butler's Patternist series covers similar ground, but that involves alien parasites, so may not quite be what you're looking for.

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As others have already said, the process of species-divergence would take far too long, even if you assume that nobody in either population would stray. And even if two populations did evolve into separate species, there would be no guarantee that they still couldn't interbreed.

However, if you're more interested in the scenario where two populations cannot interbreed -- but they are not necessarily separate species -- then I propose the following idea:

There are certain foods that are considered "class-specific" foods. That is, some foods are considered to be appropriate only for the upper class, and some foods are considered appropriate for the lower class.

It just so happens that a certain "upper-class" food is poisonous to people who carry a certain gene. It also so happens that a certain "lower-class" food is poisonous to people who carry a different gene. What will happen over time?

Over time, those in the upper class with the problem gene will die out, making it less likely that that gene is passed to their descendants. And the same thing will happen with the lower class, in that their own problem gene eventually disappears from their population.

So the upper class no longer has gene A, and the lower class no longer has gene B.

Now, suppose these two genes are crucial to reproduction; every member of this species has to have exactly one of gene A or gene B to survive childbirth and/or conception. Having both (or neither) won't work, in that a child can't live with both the A and B genes.

Assuming that each parent must give their A or B gene to the child, the fact that a mixed-class child would necessarily have both would spell its doom.

In this way you can have two separate populations of the same species that could not interbreed. Your fictional characters may or may not know about genetics (whether they do is up to you), but if they don't, then they could consider the offspring-less unions of mixed classes to be proof that classes are not supposed to be mixed.

Now you might ask: Hey, if the upper classes originally had gene A and then lost it, wouldn't it necessarily mean that they had both genes A and B at one time a long time ago? (And the same problem with the lower classes.)

Yes, that could be. But there are work-arounds you can consider: Maybe there once was a C gene (that is now gone) that allowed for mixing of A and B genes during olden times. Or maybe the A and/or B gene(s) evolved a bit to be slightly different today than it was centuries ago. Or maybe these fictional people aren't quite human like we are, and follow slightly different rules of evolution.

Even in humans, there are certain genetic conditions (such as Hemolytic disease of the newborn) that can cause difficult childbirth. So it's not too much of a stretch to figure out a case where the sexual union of people of two different populations (of the same species) can cause problems in reproduction.

In the end, it's up to you.

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